The ZnS (Ag) scintillation detector is relatively durable and inexpensive, and hence, is most extensively used for the radiation control of α-rays in nuclear facilities and the like. The ZnS (Ag) scintillator is a crystal (powder) with a trace of Ag as an activator added to ZnS, and emits scintillator light when excited by α-rays (see, for example, “Nuclear Energy Handbook (new edition)”, 3.4 Measurement of Radioactive Rays, p. 73 to 78, published on Mar. 30, 1989, by Ohm-sha). The conventional detector measures the radioactivity of α-ray emitting nuclides contained in an object to be measured merely by counting pulse signals which are obtained by amplifying the scintillator light.
The above described ZnS (Ag) scintillation detector has a problem that the counting is affected by α-rays from natural radioactive nuclides (progenies of radon) in a period that static electricity is easily generated, and in an ill-ventilated place. This is because the conventional ZnS (Ag) scintillation detector is not provided with a pulse height discrimination function, and hence, is unable to discriminate between α-rays from a radioactive material to be detected and α-rays from progenies of radon, whereby the conventional ZnS (Ag) scintillation detector counts all the α-rays.
In order to properly perform the contamination control in nuclear facilities and the like, it is necessary to eliminate the effect of natural radioactive nuclides as much as possible, and to perform the counting of α-rays only from the radioactive material to be detected. However, this is difficult to be effected in the conventional ZnS (Ag) scintillation detector without the pulse height discrimination function.